blur/fake-blur: fix various issues when using the wallpaper as the image

[taj-ny]

This fixes the following issues:

• Fake blur doesn't work on screens other than the primary one
• The image isn't scaled properly
• The image doesn't change when the wallpaper does
• The image may be black if a window had requested blur before the wallpaper appeared
• Fake blur doesn't work properly on X11 with multiple